1. Field of the Invention
The present invention relates generally to a system and method for detecting an engine knocking for an internal combustion engine and, more particularly, relates to a system and method for detecting the knocking for the internal combustion engine according to frequency signal components of detection signals derived from knocking sensor(s) and generated on the basis of engine vibrations.
2. Description of the Background Art
A Japanese Patent Application First Publication No. Showa 58-105036 exemplifies a previously proposed ignition timing controlling system which avoids an occurrence of knocking speedily by correcting an ignition timing for respective spark plugs installed in respective engine cylinders upon detection of knocking since an ill effect of knocking impulses on intake and exhaust gas valves and piston(s) is given to the respective cylinders as well as the reduction of output power.
The knocking detection due to the correction of ignition timing upon the generation of knocking occurrence is carried out as follows:
That is to say, knock sensors such as those constituted by piezoelectric elements are attached onto respective cylinder blocks of the engine and outputs detection signals indicating engine pressure vibration levels. The detection signals from the knock sensors are input to band pass filter circuits so as to pass the signal components whose frequencies are placed in the vicinity to central frequencies particular to the engine knocking, and, after half rectifications thereof are carried out, for a predetermined integration interval by means of an integrator (for example, ATDC (After Top Dead Center 10.degree..about.60.degree.) the half rectified detection signals are integrated. The integrated values are input to a microcomputer via an A/D converter. The microcomputer determines whether the knocking occurs for the respective engine cylinders on the basis of differences between the integrated values at the time of predicted occurrence of knockings and those at the time of no predicted occurrence of knockings (machine vibration levels).
In addition, another previously proposed engine knocking detecting system teaches that the knock sensors having resonance characteristics for a single frequency may be used in place of the band pass filter circuits described above.
It is generally known that vibrating waveforms of the knockings have such tendencies for their amplitudes to increase locally over predetermined intervals.
Therefore, in a region wherein the occurrence of the knocking is determined according to the integrated value only for a particular frequency signal component, it may erroneously be detected that the knocking has occurred even if an average of the individual vibration levels during the detection interval is increased due to any factor except the knocking.
Particularly, in a ease where the difference between the integrated values at the times of occurrence of knocking and of no occurrence of knocking cannot sufficiently be secured in an engine operating region, a probability of erroneous detection of knockings becomes increased since no clear difference is present when the knocking occurs or when no knocking occurs, in the above-described case.